Conventionally, it is known that partially saponified polyvinyl alcohol polymers are used as a dispersion stabilizer in suspension polymerization of vinyl compounds represented by vinyl chloride.
Dispersion stabilizers for suspension polymerization of vinyl compounds are required to satisfy ideally all the following required performance [1] to [4], even if used in small amounts. The necessary performance desired is that: with respect to the vinyl polymer particles to be obtained, [1] plasticizer absorptivity is high so that processing is easy, [2] removal of the residual monomer components such as a vinyl compound is easy, [3] the number of coarse particles is small, [4] particle size thereof is as uniform as possible so that formation of fish-eyes and the like can be prevented.
In response to these requirements, for example, a method is proposed in which polyvinyl alcohol having a low polymerization degree and a low saponification degree is used as a dispersing aid in combination with the dispersion stabilizer for suspension polymerization of vinyl compounds.
JP 4(1992)-154810 A describes use of an aqueous dispersion in which the dispersoid is a vinyl ester polymer that includes an amino group, an ammonium group, a carboxyl group or a sulfonic acid group in its side chain, and that has a saponification degree of 70 mol % or less.
JP 10(1998)-259213 A describes use of an aqueous solution of a partially saponified vinyl ester resin that includes an oxyalkylene group, a carboxyl group, a sulfonic acid group, an amino group or an ammonium group, and that has a saponification degree of 70 mol % or less.
JP 53(1978)-6392 A discloses a process of suspension-polymerizing vinyl chloride using a nonionic emulsifier and partially saponified polyvinyl acetate having a saponification degree of 40 to 55 mol % and a 4% solution viscosity in a mixed solvent of isopropanol and water (isopropanol:water=1:1) of 5 to 15 mPa s (which is equivalent to an average polymerization degree of 74 to 80).
JP 56(1981)-167745 A discloses an aqueous dispersion in which the dispersion stabilizer is polyvinyl alcohol having a saponification degree of 65 mol % or more and an average polymerization degree of 100 to 3000, and the dispersoid is polyvinyl ester having a saponification degree of 20 to 65 mol % and an average polymerization degree of 1000 or less. According to the description, the aqueous dispersion can be used as a dispersion stabilizing aid for suspension polymerization of vinyl chloride.
WO 91/15518 A discloses an aqueous dispersion in which a terminal ion-modified polyvinyl ester polymer having a saponification degree of 60 mol % or less and an average polymerization degree of 50 to 3000 is used as a dispersing aid, and use of the aqueous dispersion as a dispersion stabilizer for suspension polymerization of vinyl monomers, such as vinyl chloride.
JP 52(1977)-110797 A discloses a dispersing aid for suspension polymerization of vinyl chloride composed of partially saponified polyvinyl alcohol with a block structure, having a saponification degree of 30 to 65 mol % and a polymerization degree of 60 to 6000.
JP 5(1993)-345805 A discloses a dispersing aid for suspension polymerization of a vinyl compound composed of an aqueous dispersion in which the dispersoid is a polyvinyl ester polymer having a saponification degree of 60 mol % or less, an average polymerization degree of 4000 or more, and an average particle size of 100 μm or less.
JP 7(1995)-62006 A discloses a process for producing a vinyl chloride polymer in which partially saponified polyvinyl alcohol having a saponification degree of 70 to 85 mol % and an average polymerization degree of 1500 to 3000 is used as a dispersion stabilizer, while partially saponified polyvinyl alcohol having a saponification degree of 20 to 55 mol % and an average polymerization degree of 100 to 600 is used as a dispersing aid, and reaction is started after dissolving a part or all of the dispersing aid in a vinyl chloride monomer.
JP 9(1997)-77807 A discloses a dispersing aid for suspension polymerization of a vinyl compound composed of a vinyl ester polymer that includes a hydroxy alkyl group in its side chain, and that has a saponification degree of less than 65 mol %.
JP 10(1998)-168128 A discloses a polyvinyl alcohol polymer that is useful, for example, as a dispersing aid for suspension polymerization of a vinyl compound, that includes an ionic group at its one terminal, and that has a saponification degree of 10 to 85 mol % and a polymerization degree of 50 to 3000.
JP 9(1997)-100301 A discloses a dispersing aid for suspension polymerization of a vinyl compound composed of an aqueous solution containing 10 to 50 wt % of a vinyl ester resin with a content of 0.5 to 10 mol % of an oxyalkylene group and a saponification degree of 70 mol % or less.
JP 10(1998)-152508 A discloses a dispersing aid for suspension polymerization of a vinyl compound composed of a vinyl ester polymer that contains 10 mol % or less of an ionic group in its side chain or at its terminal, and that has a pH of 4.0 to 7.0 in a state of an aqueous dispersion and a saponification degree of 60 mol % or less.
JP 9(1997)-183805 A discloses a dispersing aid for suspension polymerization of a vinyl compound composed of a polyvinyl ester polymer that includes 0.01 to 0.3 mol % of a sulfonic acid group and 0.05 to 1.0 mol % of a carboxyl group in its side chain or at its terminal with the mole ratio of the two being in the range from 0.1 to 0.5, and that has a saponification degree of 60 mol % or less.
JP 8(1996)-109206 A discloses a dispersion stabilizer for suspension polymerization of a vinyl compound composed of a polyvinyl alcohol polymer (A) that includes a mercapto group at its terminal and that has a saponification degree of 60 mol % or more, and a polyvinyl ester polymer (B) that has a saponification degree of less than 60 mol %, with the weight ratio of the component (A) to the component (B) expressed as (A)/(B) being in the range from 40/60 to 95/5.
JP 8(1996)-259609 A discloses a dispersion stabilizer for suspension polymerization of a vinyl compound composed by mixing modified polyvinyl alcohol (A) that includes 0.01 to 10 mol % of one selected from a sulfonic acid group, an amino group, an ammonium group, a carboxyl group and a cationic group, and that has a content of ethylene units of 1 to 24 mol % and a saponification degree of more than 80 mol %, and a polyvinyl alcohol polymer (B) having a saponification degree of 60 to 95 mol % and a polymerization degree of 600 or more, with a mixing ratio of (A) component/(B) component being in the range from 1/9 to 8/2 by weight ratio.
JP 2002-37807 A discloses a dispersion stabilizer for suspension polymerization of a vinyl compound composed of a polyvinyl alcohol polymer (A) having a saponification degree of 60 mol % or more and a polymerization degree of 600 or more, and a modified polyvinyl alcohol polymer (B) having a content of ethylene units of 0.5 to 20 mol %, a saponification degree of 20 to 60 mol % and a polymerization degree of 100 to 600.
However, even in the case of using dispersion stabilizing aids for suspension polymerization described in these patent documents, the required performance [1] to [4] has not been satisfied thoroughly enough.